I have a test environment, I had 2 Users and shared mailbox in Exchange 2007, I moved the shared mailbox to Exchange 2013, now bot users cannot access the mailbox.
UserA, UserB and MBX1 are all in Exchange 2007
UserA and UserB have full access to MBX1
UserA and UserB can opem MBX1 (added to outlook)
MBX1 moved to Exchange 2013
UserA and UserB can't access MBX1 anymore